Head-up display and program

ABSTRACT

An image synthesizer apparatus for vehicle includes an image generator and an error detector. From multiple cameras arranged to a vehicle so that an imaging region of each camera partially overlaps with an imaging region of an adjacent camera, the image generator acquires images of areas allocated to the respective cameras, and synthesizes the acquired images to generate a synthetic image around the vehicle viewed from a viewpoint above the vehicle. The error detector detects errors in the cameras. When the error detector detects a faulty camera in the cameras, the image generator acquires, from the image captured by the camera adjacent to the faulty camera, an overlap portion overlapping with the image captured by the faulty camera, uses the overlap portion to generate the synthetic image, and applies image reinforcement to the overlap portion.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. patentapplication Ser. No. 14/900,560, filed on Dec. 21, 2015, which is anational stage application of PCT Application No. PCT/JP2014/003494,filed on Jul. 1, 2014, which claims priority to Japanese PatentApplication No. 2013-138971 filed on Jul. 2, 2013, the disclosure ofwhich is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a head-up display and a program fordisplaying information with a front windshield of an automobile.

BACKGROUND ART

There is a proposed driving assistance apparatus for automobile (seePatent Literature 1 for example) in which a display unit with ahorizontally-long display screen is installed below the front windshieldand a display-targeting object ahead of the automobile (specifically, inits direction) is displayed on the display screen.

This proposed apparatus, based on a positional relationship between theautomobile and the display-targeting object, estimates an actualdirection to the display-targeting object relative to an eye pointposition of the driver and sets a display position of thedisplay-targeting object to a position in the display screenintersecting with this direction.

When this technology is applied to a head-up display (also referred toas HUD hereinafter), a predetermined guide image (a graphic or the likerepresenting a display-targeting object) can be overlaid at a positionof the display-targeting object in the driver's field of view.

Concerning a head-up display, the inventors of the present applicationhave found out the following.

In the above proposed technology, the display is enlarged as thedisplay-targeting object approaches the automobile. When thedisplay-targeting object goes out of a display region in a leftdirection or a right direction, an icon is displayed at a left edge or aright edge of the display screen.

Accordingly, when the above proposed technology is applied to a head-updisplay to enable the overlay display on the display-targeting object,it may provide a troublesome feeling to the driver.

Specifically, when the guide image, which is overlay-displayed on thedisplay-targeting object, is enlarged in accordance with approach to thedisplay-targeting object, it decreases a forward visibility of theautomobile and causes troublesome.

Moreover, when a position of the display-targeting object goes out ofthe display region of the front windshield in the driver's field ofview, the guide image suddenly disappears and an icon is displayed at aleft edge or a right edge of the display screen. This is a troublesomedisplay for the driver.

In particular, when the display-targeting object (e.g., a trafficsignal) moves from an upper part of the front windshield to a rear side,the display of the icon representing its direction at a left edge or aright edge of the display screen provides an awkward feeling to thedriver.

PRIOR ART LITERATURES Patent Literature

Patent Literature 1: JP 2006-23277A

SUMMARY OF INVENTION

It is an object of the present disclosure to enable a head-up display,which displays a predetermined guide image by overlaying it on adisplay-targeting object in a field of view from an cabin of a vehicle,to change the guide image display without providing a troublesomefeeling to a driver when the display-targeting object goes out of adisplay region.

In an example of the present disclosure, a head-up display comprises: adisplay unit that displays an image to a display region configured in afront windshield of an automobile; an automobile information acquirer; aforward environment information acquirer; a display-targeting objectdetector; and a display controller.

The automobile information acquirer acquires automobile informationrepresenting a traveling state of the automobile. The forwardenvironment information acquirer acquires forward environmentinformation representing an environment forward of the automobile in atraveling direction. The display-targeting object detector, based on theautomobile information and the forward environment information, detectsa display-targeting object on which guide information is to be displayedto the display region of the front windshield.

The display controller generates a guide image for overly-display of theguide information to the front windshield and performs overlay-displayof the guide image to the front windshield with the display unit so thatwhen the display-targeting object detected by the display-targetingobject detector is viewed from an inside of a cabin of the automobile,the guide information and the display-targeting object overlap eachother.

When a position of the display-targeting object changes and goes out ofa predetermined display-setting area, the display controller darkens theguide image or changes over the guide image from a normal image to asimplified image which is smaller than the normal image.

According to the head-up display in the disclosure, when a position ofthe display-targeting object, which is a position viewed from the insideof the automobile cabin, goes out of a predetermined display-settingarea, the guide image overlay-displayed on the display-targeting objectdoes not suddenly disappears. It is possible to prevent this displaychange from providing a troublesome feeling to a driver.

Additionally, although the guide image is darkened or changed over to asmaller simplified image than the normal image when the position of thedisplay-targeting object changes and goes out of the predetermineddisplay-setting area, Its display position still corresponds to theposition of the display-targeting object and thus it is possible toprevent this display change of the display-targeting object fromproviding a troublesome feeling to a driver.

Therefore, the head-up display in the disclosure is excellent inusability for an occupant such as a driver and the like.

The automobile information acquirer, the forward environment informationacquirer, the display-targeting object detector and the displaycontroller in the head-up display may be implemented by a programexecuted by a computer.

In this configuration, the computer can function as the above units inthe disclosure when the program is stored in, for example, acomputer-readable storage medium such as DVD-ROM, CD-ROM and hard diskand loaded to the computer on an as-needed basis or when the computeracquires the program via a communication line.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an outline configuration of ahead-up display of an embodiment;

FIG. 2 is a flowchart illustrating a display control process performedby a control unit;

FIG. 3 is a flowchart illustrating a vehicle information acquisitionprocess in FIG. 2;

FIG. 4 is a flowchart illustrating a forward environment informationacquisition process shown in FIG. 2;

FIG. 5 is a display parameter setting process shown in FIG. 2;

FIG. 6 is a diagram illustrating a map for setting a luminancecorrection value according to distance;

FIG. 7 is a diagram illustrating a luminance change of guide image whena display-targeting object is large;

FIG. 8 is a diagram illustrating a map for setting a luminancecorrection value according to display position;

FIG. 9 is a diagram illustrating a luminance change of a guide imageaccording to display position;

FIG. 10 is a flowchart illustrating a first modification of a displayparameter setting process;

FIG. 11 is a diagram illustrating a guide image change according to adisplay parameter setting process of FIG. 10;

FIG. 12 is a flowchart illustrating a second modification of a displayparameter setting process;

FIG. 13 is a diagram illustrating a changeover manner of a guide imageaccording to a display parameter setting process of FIG. 12; and

FIG. 14 is a diagram illustrating a guide image change according to adisplay parameter setting process of FIG. 12.

EMBODIMENTS FOR CARRYING OUT INVENTION

Embodiments will be described with the drawings.

As shown in FIG. 1, a head-up display (HUD) 2 of the present embodimentincudes a display unit 10 and a control unit 30. The display unit 10emits a display light for image display to a front windshield of anautomobile, thereby visually displaying a virtual image so that thevirtual image is overlaid in the actual field of view ahead of theautomobile. The control unit 30 controls the display unit 10.

The control unit 30 includes a microcomputer with a CPU, a ROM, a RAMand the like, and generates various guide images and displays the guideimages (virtual images) with the front windshield via the display unit10.

The control unit 30 is connected with a meter 12, a GPS receiver 14, amap database 16, a vehicle speed sensor 18, a radar apparatus 20, aroad-vehicle communication 22, a display setting switch (SW) 24, acamera 26 and the like in order to acquire a variety of informationneeded to generate the guide images.

The control unit 30 is further connected with in-vehicle apparatusessuch as an ACC ECU 4 and a navigation apparatus 6 via a communicationline 8 which constitutes an in-vehicle LAN. By communicating with thesein-vehicle apparatuses, the control unit 30 can acquire information fromother in-vehicle apparatuses.

The meter 12 includes, for example, a color display with a displayscreen such as a liquid crystal display and a needle type indicator. Themeter 12 is provided in an instrumental panel in front of a driver seatand displays various vehicle states. The control unit 30 displays anoperating state of the HUD 2 on the meter 12, and acquires the vehiclestate from the meter 12 on an as-needed basis.

The GPS receiver 14 receives electromagnetic wave transmitted from GPSsatellites and detects vehicle position, vehicle speed, accelerationetc. The control unit 30 acquires these kinds of information from theGPS receiver 14.

The GPS receiver 14 may receive electromagnetic wave transmitted fromGPS satellites operated by the United States of America and may detectvehicle position, vehicle speed, acceleration etc. In addition toreceiving the electromagnetic wave transmitted from the GPS satellites,the GPS receiver 14 may receive the electromagnetic wave transmittedfrom other satellites for satellite positioning systems (GLONASS,Galileo, Quasi-zenith satellites) and detect vehicle position, vehiclespeed, acceleration etc. Alternatively, instead of the GPS receiver 14,a device for receiving the electromagnetic wave transmitted fromsatellites for satellite positioning systems other than GPS anddetecting vehicle position, vehicle speed, acceleration etc. may beused.

The map database 16 stores map data (node data, link data, cost data,road data, topographical data, mark data, intersection data, stop pointdata, facility data etc.) and is used to provide the stored map data tothe control unit 30.

The vehicle speed sensor 18 is attached to a tire wheel or a propellershaft and generates a pulse signal according to its rotation and is usedto detect travel speed of the vehicle (automobile speed) based on timeintervals of the pulse signals.

The radar apparatus 20 transmits a radar wave toward the front of thevehicle and is used to detect a distance to a vehicle ahead (precedingvehicle) and a relative speed.

The road-vehicle communicator 22 performs wireless communication withon-road equipment located near a travel route of the vehicle and is usedto acquire road traffic information (road congestion information,traffic regulation information etc.) from the on-road equipment.

The display setting SW 24 is used to set a display mode of the HUD 2such as a guide image type displayed to the automobile front windshieldvia the display unit 10 and includes a switch manipulated by a user.

The camera 26 captures an image in a vehicle traveling direction(forward environment image). The captured image is used for the controlunit 30 to detect a display-targeting object, which is an object forwhich the guide image is displayed.

The ACC ECU 4 is an electronic control unit for performing ACC(auto-cruise control). The navigation apparatus 6 displays a road map ona display in front of the driver's seat and performs travel routeguidance.

Conventional elements may be used for the above elements. Next, adisplay control process performed by the control unit 30 will bedescribed.

The microcomputer of the control unit 30 executes a program stored in astorage medium such as the ROM to implement the display control process.

As shown in FIG. 2, when starting the display control process, thecontrol unit 30 first performs S100 (S denotes step) to acquirestart-up-related information of the in-vehicle apparatuses connected viathe communication line 8 and acquire the operating mode of the HUD 2 setvia the display setting SW 24.

For example, at S100, the control unit 30 acquires ON/OFF of the routeguidance of the navigation apparatus 6, ON/OFF of the ACC ECU4, ON/OFFof warning about speed limit/road traffic signal etc. As a result, thecontrol unit 30 can identify a type of display-targeting object forwhich the guide image is to be overlay-displayed through the frontwindshield.

Subsequently, at S200, the control unit 30 performs a vehicleinformation acquisition process to acquire a driving state of thevehicle and a surrounding condition. At S300, the control unit 30performs a forward environment information acquisition process toacquire forward environment information such as a road condition aheadof the vehicle and a preceding vehicle.

At S400, based on the information acquired at S100 to S300, the controlunit 30 determines whether or not the display-targeting object, forwhich the guide image should be overlay-displayed through the frontwindshield, is present. The display-targeting object may be a roadtraffic signal, a road sign, an advertisement, a preceding vehicle, apedestrian or the like.

When the control unit 30 determines at S400 the display-targeting objectis present, the control unit 30 proceeds to S500 and sets at displayparameter corresponding to the display-targeting object such as a typeof guide image, a display position, and a display manner.

At S600, based on the display parameter set in the display parametersetting process at S500, the control unit 30 performs a display processand then proceeds to S100. In the display process, the guide image 4 fordisplay is generated and outputted to the display unit 10 so that, whenviewed from the eye point of the driver, the guide image (specifically,a graphic corresponding to the display-targeting object) is displayed atthe position that matches the position of the display-targeting objectin the windshield.

When determining at S400 that there is no display-targeting object, thecontrol unit 30 proceeds to S700 and determines whether or not the guideimage is presently displayed. When the guide image is presentlydisplayed, the control unit 30 proceeds to S800 to perform a non-displayprocess to implement non-display of the guide image and proceeds toS100. When the guide image is not presently displayed, the control unit30 proceeds to S100.

In the non-display process at S800, the luminance of the guide image isgradually decreased with a given damping time constant and the displayis gradually cleared, so that the guide image does not instantaneouslydisappear (in order words, a sudden luminance change is avoided).

The vehicle information acquisition process at S200 is performed, forexample, in a manner illustrated in FIG. 3.

Specifically, in the vehicle information acquisition process, thecontrol unit 30 acquires vehicle speed using the vehicle speed sensor18, the GPS receiver 14 or the like at S210. The vehicle speed may beacquired by integration of the acceleration detected by an accelerationsensor (not shown). The vehicle speed may be acquired from the GPSreceiver 14. Alternatively, the vehicle speed may be acquired via thecommunication line 8 from another in-vehicle apparatus having a vehiclespeed detection function.

Subsequently, at S220, the control unit 30 acquires acceleration of thevehicle (vehicle acceleration) from an acceleration sensor (not shown),the GPS receiver 14, or another in-vehicle apparatus connected via thecommunication line 8.

At S230, the control unit 30 acquires a coordinate point of the vehicle(vehicle position) from the GPS receiver 14, an inertial navigationapparatus (not shown), a matching of these detection results with themap database 16, a navigation apparatus 6 connected via thecommunication line 8, or the like.

Subsequently, at S240, the control unit 30 acquires information on avehicle posture (angular velocities about X, Y, X axes, angularaccelerations, directions of respective axes) from a gyro sensor (notshown), a geomagnetic sensor (not shown), or another in-vehicleapparatus connected via the communication line 8.

At S250, the control unit 30 acquires a steering angle of the vehiclefrom a steering angle sensor (not shown) attached to a steering columnor a steering linkage or from another in-vehicle apparatus connected viathe communication line 8.

Subsequently, at S260, the control unit 30 acquires braking pressure (inother words, brake force) from a pressure sensor (not shown) provided toa brake-related oil pressure system or from another in-vehicle apparatus(ABS ECU etc.) connected via the communication line 8.

At S270, the control unit 30 acquires an opening degree of a throttlevalve (throttle opening degree) from a throttle position sensor (notshown) provided to an engine of the vehicle or from another in-vehicleapparatus (engine ECU etc.) connected via the communication line 8.

At S280, the control unit 30 acquires illuminance outside the vehicle(external illumination) from a luminance sensor (not shown) or fromanother in-vehicle apparatus (e.g., air conditioner ECU etc.) connectedvia the communication line 8.

As described above, the vehicle information acquisition process acquiresthe vehicle information such as the vehicle seed, the vehicleacceleration, the vehicle position, the vehicle posture, the steeringangle, the braking pressure, the throttle opening degree, externalillumination etc. In the vehicle information acquisition process, anorder of S210 to S280 may be changed and some of S210 to S280 may beomitted.

The forward environment information acquisition process at S300 isperformed in a manner illustrated in, for example, FIG. 4.

Specifically, in the forward environment information acquisitionprocess, the control unit 30 first acquires a vehicle-to-vehicledistance between the vehicle and the preceding vehicle from the radarapparatus 20 at S310. Subsequently, at S320, the control unit 30acquires speed of the preceding vehicle from the radar apparatus 20. AtS330, the control unit 30 acquires positional information of thepreceding vehicle relative to the vehicle (azimuth, and elevation anddepression) from the radar apparatus 20.

These kinds of information may be acquired via the communication line 8from another in-vehicle apparatus having the same function as the radarapparatus.

Subsequently, at S340, the control unit 30 acquires positionalinformation (azimuth, elevation/depression angle, distance) of a trafficsign ahead of the vehicle based on image processing on the imagecaptured by the camera 26, the vehicle positional information acquiredat S230, the map data stored in the map database 16, and the like.

Subsequently, at S350, the control unit 30 acquires positionalinformation (azimuth, elevation/depression angle, distance) of a roadtraffic signal ahead of the vehicle based on image processing on theimage captured by the camera 26, the vehicle positional informationacquired at S230, the map data stored in the map database 16, and thelike.

At S340 and S350, the control unit 30 may use the navigation device 6connected via the communication line 8 to acquire the positionalinformation of a traffic sign and a road traffic signal.

In the above way, the forward environment information acquisitionprocess acquires the vehicle-to-vehicle distance to the precedingvehicle, the speed of the preceding vehicle, the positional informationof the preceding vehicle, the positional information of the road sign,and the positional information of the road traffic signal.

In the forward environment information acquisition process, an order ofS310 to S350 may be changed. Some of S310 to S350 may be omitted.

The display parameter setting process at S500 is performed in a mannerillustrated in FIG. 5.

In the display parameter setting process, the control unit 30 firstdetermines whether or not a size of the display-targeting object islarger than a predetermined threshold TH_VAL[m].

When the display-targeting object is larger than the thresholdTH_VAL[m], the control unit 30 proceeds to S512 and sets a luminancecorrection value for correcting the luminance of the guide imageoverlay-displayed on the display-targeting object.

The correction of the luminance is made by multiplying a referenceluminance by the luminance correction value. The luminance correctionvalue is set using a map having characteristics shown in FIG. 6.

When the display-targeting object is larger than the thresholdTH_VAL[m], the luminance of the guide image is set according to thedistance between the vehicle and the display-targeting object, so thatthe larger the distance, the smaller the luminance, and vice versa.

As shown in FIG. 7, when the display-targeting object is larger than thethreshold TH_VAL[m], the luminance of the guide image overlay-displayedon the display-targeting object decreases with increasing distancebetween the subject vehicle and the display-targeting object and withincreasing apparent size of the display-targeting object viewed throughthe front windshield. In other words, the guide image is darker as thedistance between the subject vehicle and the display-targeting object issmaller and as the apparent size of the display-targeting object viewedthrough the front windshield is larger.

In the FIG. 7, the HUD display region refers to a display region inwhich the display unit can display the guide image (figures in FIG.) tothe front windshield. The sensor detectable region refers to a region inwhich the forward environment informant acquisition process can detectthe preceding vehicle, the traffic sign, the road traffic signal and thelike.

When determining at S511 that the display-targeting object is smallerthan or equal to the threshold TH_VAL[m], the control unit 30 proceedsto S513 and determines whether or not, when viewed from the eye point ofthe driver, the display-targeting object (specifically, the guide imageoverlay-displayed on the display-related object) is within in the area Aillustrated in FIG. 8.

The area A refers to a display-setting area, which is designated so asto be smaller than the HUD display region.

When the display-targeting object is within the area A, the control unit30 proceeds to S514 in which the luminance correction value for normaldisplay is set as the luminance correction value of the guide imageoverlay-displayed on the display-targeting object.

When the display-targeting object is out of the area A, the control unit30 proceeds to S514 in which the luminance correction value is setaccording to an amount of the departure of the guide image from the areaA.

As shown in FIG. 8, a map for setting the luminance correction value isdefined such that the luminance correction value decreases withincreasing amount of departure from the area A and that the luminancecorrection value has “0” at an edge of the HUD display region.

As a result, as shown in FIG. 9, when the size of the display-targetingobject (traffic sign in the drawing) is not larger than the thresholdTH_VAL[m], the luminance of the guide image overlay-displayed on thedisplay-targeting object is set to a normal luminance for normal displayif the position of the display-targeting object viewed from the driveris within the area A and the luminance of the guide image is correctedto be a smaller value if the display-targeting object moves out of thearea A.

Accordingly, the guide image gets darker as the position of thedisplay-targeting object viewed from the driver moves to an outside ofthe area A. At the end of the HUD display region, its luminance reaches“0” and the guide image disappears.

After setting the luminance correction value at S512, S514, S515, thecontrol unit 30 proceeds to S516 and sets the display position, size anddisplay luminance of the guide image overlay-displayed on thedisplay-targeting object and ends the display parameter setting process.

At S516, based on the direction and distance to the display-targetingobject and the posture direction, speed, acceleration and like of thevehicle, the control unit 30 sets the display position and size of thedisplay-targeting object so that the displayed guide image matches theforward environment movement. The display luminance of the guide imageis calculated by setting the reference luminance based on the externalilluminance acquired at S280 and by multiplying the reference luminanceby the luminance correction value set at S512, S514, and S515.

In the above-illustrated embodiment, the guide image isoverlay-displayed on the display-targeting object. In doing so, when thedisplay-targeting object is larger than the threshold, the displayluminance of the guide image decreases with decreasing distance betweenthe subject vehicle and the display-targeting object (in other words,with increasing display area of the guide image), whereby the guideimage darkens.

When the display-targeting object is smaller than or equal to thethreshold and is out of the area A of the display region, the displayluminance of the guide image decreases according to its departureamount, whereby the guide image darkens.

Therefore, when the display-targeting object is out of the displayregion of the HUD 2 on the front windshield, the HUD 2 of the presentembodiment can prevent the guide image from suddenly disappearing andprevent this display change from providing a troublesome feeling to thedriver.

When the display-targeting object is smaller than or equal to thethreshold and the position of the display-targeting object is moves inan outside of the area A, the guide image is gradually darken. Thisprevents the luminance change of the guide image from providing atroublesome feeling to the driver.

Although one embodiment of the present disclosure has been illustrated,embodiments of the present disclosure are not limited to theabove-illustrated embodiment and can have various forms.

In the above embodiment, the luminance of the guide imageoverlay-displayed on the display-targeting object is changed accordingto the size or position of the display-targeting object viewed from thedriver, so that the troublesome feeling of the driver provided by thedisplay of the guide image is reduced.

In another configuration, the shape of the guide image overlay-displayedon the display-targeting object may be changed. This can also reduce thetroublesome feeling of the driver provided by the display of the guideimage

A display parameter setting process for this configuration will bedescribed as a first modification and a second modification.

(First Modification)

As shown in FIG. 10, in the display parameter setting process accordingto the first modification, the control unit 30 first determines at S521whether or not, when viewed from the driver, the position of thedisplay-targeting object is within the HUD display region.

When determining at S521 that the position of the display-targetingobject is within the HUD display region, the control unit 30 proceeds toS520 and determines whether or not, when viewed from the driver, theposition of the display-targeting object is within an area B.

As shown in FIG. 11, the area B is a pre-designated display-settingarea, which is inside the HUD display region and is smaller than the HUDdisplay region.

When determining at S522 that the position of the display-targetingobject is within the area B, the control unit 30 sets a normal displayto the display-targeting object (road sign in the drawing) to overlay apreset normal guide image (rectangular in the drawing), as shown in FIG.11.

When determining at S522 that the position of the display-targetingobject is not within the area B, the control unit 30 proceeds to S524and performs a display setting to display the normal guide image and anicon which acts as a simplified image indicating a direction to thedisplay-targeting object, as shown in FIG. 11.

When determining at S521 that the position of the display-targetingobject is out of the HUD display region, the control unit 30 performs adisplay setting to display only the icon at the edge position from whichthe display-targeting object moves out of the HUD display region, sothat the direction to the display-targeting object is perceivable.

When setting the normal display or performing the display setting todisplay the normal guide image and the icon or performing the displaysetting to display only the icon at S523, S524 or S525, the control unit30 sets the display position, size and display luminance of the guideimage in accordance with the set display content, and ends the displayparameter setting process.

In the first modification, when the display-targeting object is withinthe area B when viewed from the driver seat, the normal guide image isoverlay-displayed on the display-targeting object. When thedisplay-targeting object is out of the area B but within the HUD displayregion, the icon is overlay-displayed for the display-targeting objectin addition to the normal guide image. When the display-targeting objectis out of the HUD display region, only the icon is displayed at the edgeof the HUD display region.

Therefore, in the first modification, even when the display-targetingobject viewed by the driver goes out of the display region of the HUD 2on the front windshield, the guide image does not suddenly disappear.The troublesome feeling of the driver due to this display change isprevented.

Moreover, because the icon indicating the direction to thedisplay-targeting object is displayed at the edge of the HUD displayregion when the display-targeting object moves to the outside of the HUDdisplay region, the driver can recognize the direction to thedisplay-targeting object.

(Second Modification)

As shown in FIG. 11, in the display parameter setting process accordingto the second modification, the control unit 30 first determines at S531whether or not a distance from the eye point of the driver to thedisplay-targeting object (preceding vehicle in the drawing) is smallerthan a designated distance (see FIG. 13). The designated distance is adistance that enables the guide image to be overlay-displayed on thedisplay-targeting object via the display unit 10 (see FIG. 13).

In consideration of the display image distance (see FIG. 13) and humaneye's power of accommodation, this setting distance is pre-designated tosuch a distance in which the HUD image does not appear to get stuck insuch a display-targeting object as a preceding vehicle.

When the distance to the display-targeting object is greater than orequal to the designated distance, the control unit 30 proceeds to S532and sets the display position, size and display luminance of the guideimage for performing the normal display of the guide image, and thenends the display parameter setting process.

When the distance to the display-targeting object is smaller than thedesignated distance, the control unit 30 proceeds to S533 and stops thenormal display of the guide image and displays the icon, which is thesimplified image of the guide image, in a non-overlay-display area, andthen ends the display parameter setting process. The non-overlay-displayarea is different from the display region which is for the overlaydisplay of the guide image.

Specifically, when the guide image is overlay-displayed on the displaytarget object under a situation where, the distance to thedisplay-targeting object is smaller than the designated distance, theguide image gets stuck in the display-targeting object and provides astrange feeling to a driver.

In view of this, the second modification is such that when the distancefrom the eye point of the driver to the display-target object becomessmaller than the designated distance illustrated in FIG. 13, it isdetermined that the position of the display-targeting object goes out ofthe display-setting area, and the guide image is changed over from thenormal display illustrated in FIG. 14 to the icon display on thenon-overlay-display area.

Therefore, in the second modification, even when the vehicle approachesthe preceding vehicle serving as an ACC target and the distance to thepreceding vehicle becomes smaller than the designated distance, theguide image is displayed so as not to get stuck in the proceedingvehicle and the driver is prevented from having a strange feeling.

In FIG. 14, the non-overlay-display area is provided below the HUDdisplay region. An icon indicating the ACC control is in operation isdisplayed in the non-overlay-display area.

Because it is sufficient that this icon display notifies the driver thatthe ACC control is in operation, the non-overlay-display area may beprovided in the meter 12 and the meter 12 may display the iconindicating that ACC control is in operation.

In the above embodiments and modifications, the control unit 30performing the S200 acts as an embodiment of a vehicle informationacquirer. The control unit 30 performing the S300 acts as an embodimentof a forward environment information acquirer. The control unit 30performing the S400 acts as an embodiment of a display-targeting-objectdetector. The control unit 30 performing the S500 acts as an embodimentof a display controller.

The invention claimed is:
 1. A head-up display comprising: a displayunit that displays an image to a display region configured in a frontwindshield of an automobile; an automobile information acquirer thatacquires automobile information representing a traveling state of theautomobile; a forward environment information acquirer that acquiresforward environment information representing an environment forward ofthe automobile in a traveling direction; a display-targeting objectdetector that, based on the automobile information and the forwardenvironment information, detects a display-targeting object on whichguide information is to be displayed to the display region of the frontwindshield; and a display controller that generates a guide image foroverlay-display of the guide information to the front windshield andperforms overlay-display of the guide image to the front windshield withthe display unit so that the display-targeting object detected by thedisplay-targeting object detector is recognizable an inside of a cabinof the automobile, the guide information and the display-targetingobject overlap each other, wherein an apparent size of the guide imageis constant within the display region, the display region includes apredetermined display-setting area that is located within the displayregion, smaller than the display region and located near a center of thedisplay region, when a position of the display-targeting object iswithin the predetermined display-setting area, the display controller isset to a normal luminance for normal display, and when the position ofthe display-targeting object changes and goes out of the predetermineddisplay-setting area, the display controller decreases a displayluminance of the guide image such that the guide image becomes darker asa distance of the display-targeting object in a vertical direction and ahorizontal direction from a center of the predetermined display-settingarea increases.
 2. The head-up display according to claim 1, wherein inresponse to the position of the display-targeting object, which is theposition viewed from the inside of the cabin of the automobile, movingfrom an inside of the display region, the display controller changesluminance of the guide image so that the guide image disappears at atime the position of the display-targeting object reaches an edge of thedisplay region.
 3. A head-up display comprising: a display unit thatdisplays an image to a display region configured in a front windshieldof an automobile; and a display controller configured to control thedisplay unit, wherein the display controller includes an automobileinformation acquirer that acquires automobile information representing atraveling state of the automobile, a forward environment informationacquirer that acquires forward environment information representing anenvironment forward of the automobile in a traveling direction, adisplay-targeting object detector that, based on the automobileinformation and the forward environment information, detects adisplay-targeting object on which guide information is to be displayedto the display region of the front windshield, a guide image generatorthat generates a guide image for overlay-display of the guideinformation to the front windshield and performs overlay-display of theguide image to the front windshield with the display unit so that thedisplay-targeting object detected by the display-targeting objectdetector is recognizable from an inside of a cabin of the automobile,and a display luminance setting unit that set a display luminance of thegenerated guide image, wherein the display unit performs overlay-displayof the guide image, of which the display luminance of the guide image isset by the display luminance setting unit, on the front windshield byemitting a display light, the display region includes a predetermineddisplay-setting area that is smaller than the display region and locatednear a center of the display region, when a position of thedisplay-targeting object is within the predetermined display-settingarea, the display luminance setting unit sets the display luminance ofthe guide image for normal display, and when position of thedisplay-targeting object goes beyond the predetermined display-settingarea, the display luminance setting unit sets a display luminance of theguide image to be smaller as a distance of the display-targeting objectin a vertical direction and a horizontal direction from a center of thepredetermined display setting area increases.
 4. The head-up displayaccording to claim 3, wherein the display controller executes a programstored in a storage medium to implement control of the display unit. 5.The head-up display according to claim 4, wherein in a non-displayprocess, the display luminance of the guide image is gradually decreasedwith a given damping time constant and the display luminance of theguide image is gradually cleared.
 6. The head-up display according toclaim 5, wherein the display luminance of the guide image is calculatedby setting a reference luminance based on an external illuminance, andby multiplying the reference luminance by a luminance correction value.